Injection activities are a required practice to enhance and ensure the productivity of oil and gas fields, especially in environments where the natural production potential of the reservoir is limited (e.g. low-permeability formations). Generally, injection activities use special chemical solutions to improve oil recovery, remove formation damage, clean blocked perforations or formation layers, reduce or inhibit corrosion, upgrade crude oil, or address crude oil flow-assurance issues. Injection can be administered continuously, in batches, in injection wells, or at times in production wells.
In a majority of cases, wells that will be subject to injection activities are completed with a cemented casing across the formation of interest to assure borehole integrity and allow selective injection into and/or production of fluids from specific intervals within the formation. It is necessary to perforate this casing across the interval(s) of interest to permit the ingress or egress of fluids. Several methods are applied to perforate the casing, including mechanical cutting, hydro-jetting, bullet guns and shaped charges. The preferred solution in most cases is shaped charge perforation because a large number of holes can be created simultaneously, at relatively low cost. Furthermore, the depth of penetration into the formation is sufficient to bypass near-wellbore permeability reduction caused by the invasion of incompatible fluids during drilling and completion. The vast majority of perforated completions depend on the use of shaped charges because of the relative speed and simplicity of their deployment compared to alternatives, such as mechanical penetrators or hydro-abrasive jetting tools. However, despite these advantages shaped charges provide an imperfect solution.
FIG. 1A illustrates a perforating gun 10 consisting of a cylindrical charge carrier 14 with shaped charges 16 (also known as perforators) lowered into the well by means of a cable, wireline, coil tubing or assembly of jointed pipe 18. Any technique known in the art may be used to deploy the carrier 14 into the well casing. At the well site, the shaped charges 16 are placed into the charge carrier 14, and the charge carrier 14 is then lowered into the oil and gas well casing to the depth of a hydrocarbon bearing formation 12.
FIG. 1B depicts a blown-up view of a conventional shaped charge 16 next to a hydrocarbon bearing formation 12, as referenced in FIG. 1A. The shaped charge 16 is formed by compressing explosive powder (also known as an explosive load) 22 within a metal case 20 using a conical or parabolic metal liner 24. When the explosive powder 22 is detonated, the symmetry of the charge 16 causes the metal liner 24 to collapse along its axis into a narrow, focused jet of fast moving metal particles. Consequently, the shaped charge 16 will perforate the carrier 14, casing 26, cement sheath 28, and finally the formation 12. As the charge jet penetrates the rock it decelerates until eventually the jet tip velocity falls below the critical velocity required for it to continue penetrating.
Perforation is inevitably a violent event, pulverizing formation rock grains and resulting in plastic deformation of the penetrated rock, grain fracturing, and the compaction of particulate debris (fractured sand grains, cement particles, and/or metal particles from casing, shaped charge fragments or the disintegrating liner) into the tunnel and the pore throats of rock surrounding the tunnel. As seen in the tunnels 32 of FIG. 2, particulate debris 38 resulting from perforation can cause any number of blockages, ranging from entirely blocking an opening 34 to a tunnel 32 or substantially filling the area of the tunnel 32, for example. This debris 38 can limit the effectiveness of the created tunnel as a conduit for flow since debris inside the perforation tunnel and embedded into the wall of the tunnel may block the ingress or egress of fluids or gases. This may cause significant operational difficulties for the well operator and the debris may have to be cleaned out of the tunnels at significant cost.
FIG. 3A depicts a close-up view detailing the typical tunnel after a traditional shaped charge 16 is fired from a perforating gun 14 and into a hydrocarbon bearing formation 12 as shown in FIG. 2. As shown in FIG. 3A, the resulting tunnel 32 created through the hole 34 in the casing wall is relatively narrow. Particulate jet debris 38 and material from the formation 12 piles up at the tip 30 of the newly created tunnel 32. This compacted mass of debris 38, enlarged in FIG. 3B, at the tip 30 of the tunnel is typically very hard and almost impermeable, reducing the inflow and/or outflow potential of the tunnel and the effective tunnel depth, re (also known as clear tunnel depth). Plugged tips 30 impair flow and obstruct the production of oil and gas from the well. In addition, the particulate debris that the perforating event drives into the surrounding pore throats results in a zone 36 of reduced permeability (disturbed rock) around the perforation tunnel 32 commonly known as the “crushed zone,” which typically contains pulverized and compacted rock. The crushed zone 36, though only about one quarter inch thick around the tunnel, detrimentally affects the inflow and/or outflow potential of the tunnel 32 (commonly known as a “skin” effect.) Plastic deformation of the rock during perforation also results in a semi-permanent zone 42 of increased stress around the tunnel, known as a “stress cage”, which impairs fracture initiation from the tunnel. The perforating event is so fast that the associated rock deformation and compaction exceed the elastic limit of the rock and result in permanent plastic deformation. Along with changes in porosity and permeability, the in-situ stress in the plastically deformed rock is also substantially changed, forming the stress cage 42 extending up to several inches beyond the actual dimensions of the tunnel.
The distance a perforated tunnel extends into the surrounding formation, commonly referred to as total penetration, is a function of the explosive weight of the shaped charge; the size, weight, and grade of the casing; the prevailing formation strength; and the effective stress acting on the formation at the time of perforating. Effective penetration is the fraction of the total penetration that contributes to the inflow or outflow of fluids. This is determined by the amount of compacted debris left in the tunnel after the perforating event is completed. The effective penetration may vary significantly from perforation to perforation. Currently, there is no means of measuring it in the borehole. Darcy's law relates fluid flow through a porous medium to permeability and other variables, and is represented by the equation seen below:
          ⁢      q    =                  2        ⁢                                  ⁢        π        ⁢                                  ⁢                  kh          ⁡                      (                          -                              p                w                                      )                                      µ        ⁡                  [                                    ⁡                              (                                                      r                    e                                                        r                    w                                                  )                                      +            S                    ]                    Where: q=flowrate, k=permeability, h=reservoir height, pe=pressure at the reservoir boundary, pw=pressure at the wellbore, μ=fluid viscosity, re=radius of the reservoir boundary, rw=radius of the wellbore, and S=skin factor.The effective penetration determines the effective wellbore radius, rw, an important term in the Darcy equation for the radial inflow. This becomes even more significant when near-wellbore formation damage has occurred during the drilling and completion process, for example, resulting from mud filtrate invasion. If the effective penetration is less than the depth of the invasion, fluid flow can be seriously impaired.
To optimize the production potential of a tunnel, current methods rely on either remedial operations during or after the perforation or modification of the system configuration. For example, current procedures commonly rely on the creation of a relatively large static pressure differential, or underbalance, between the formation and the wellbore, wherein the formation pressure is greater than the wellbore pressure. These methods attempt to enhance tunnel cleanout by controlling the static and dynamic pressure behavior within the wellbore prior to, during and immediately following the perforating event so that a pressure gradient is maintained from the formation toward the wellbore, inducing tensile failure of the damaged rock around the tunnel and a surge of flow to transport debris from the perforation tunnels into the wellbore. Underbalanced perforating involves creating the opening through the casing under conditions in which the hydrostatic pressure inside the casing is less than the reservoir pressure, allowing the reservoir fluid to flow into the wellbore. If the reservoir pressure and/or formation permeability is low, or the wellbore pressure cannot be lowered substantially, there may be insufficient driving force to remove the debris. Such techniques are relatively successful in homogenous formations of moderate to high natural permeability (typically 300 millidarcies and greater), where a sufficient surge flow can be induced to clean a majority of the perforation tunnels. In such cases, the percentage of tunnels left unobstructed (also known as “perforation efficiency”) may typically be 50-75% of the total holes perforated. Furthermore, laboratory experiments indicate that the clear tunnel depth of “clean” perforations created in an underbalanced situation generally varies between 50-90% of the total penetration.
In heterogeneous formations—where rock properties such as hardness and permeability vary significantly within the perforation interval—and in formations of high-strength, high effective stress and/or low natural permeability, underbalanced techniques become increasingly less effective. Since all the tunnels are being cleaned up in parallel by a common pressure sink, perforations shot into zones of relatively higher permeability will preferentially flow and clean up, eliminating the pressure gradient before adjacent perforations shot into poorer rock are able to flow.
Since the maximum pressure gradient is limited by the difference between the reservoir pressure and the minimum hydrostatic pressure that can be achieved in the wellbore, perforations shot into low permeability rock may never experience sufficient surge flow to clean up. In such circumstances the perforation efficiency may be as low as 10% of the total holes perforated.
In low to moderate-permeability reservoirs, a hydraulic fracture is commonly used for well stimulation to bypass near-wellbore damage, increase the effective wellbore radius, and increase the overall connectivity between the reservoir and the wellbore. Execution of a hydraulic fracture involves the injection of fluids at a pressure sufficiently high to cause tensile failure of the rock. At the fracture initiation pressure, often known as the “breakdown pressure,” the rock opens. As additional fluids are injected, the opening is extended and the fracture propagates. When properly executed, a hydraulic fracture results in a “path,” connected to the well that has a much higher permeability than the surrounding formation. This path of large permeability can extend tens to hundreds of feet from the wellbore.
Perforations play a critical role in any stimulation treatment because they form the only connection between the wellbore and formation. However, arriving at an optimum perforation design can be difficult because essentially all perforated completions are damaged, as shown by way of example in FIGS. 2-3. The compacted and plastically deformed zones around the perforation can be so highly stressed that the pressure required to initiate a fracture is significantly greater than the measured fracture gradient of the unaltered rock. In extreme cases the altered rock cannot be broken down before surface equipment limitations are reached. When breakdown is possible, the induced fracture will orient itself parallel to the minimum stress acting on the formation 12. This may result in a tortuous path as depicted in FIG. 4, resulting in increased near-wellbore pressure losses, commonly known as tortuosity.
In FIG. 4, the uneven and inefficient injection and/or stimulation that results with prior art methods is seen. As chemical solutions are introduced, debris 38 prevents their introduction through plugged tunnels, causing poor coverage across the targeted formation interval. The limited number of open perforation tunnels forces fluids to find tortuous pathways around the partially blocked tunnels. Furthermore, a high percentage of blocked tunnels means that only relatively few open tunnels will be aligned with the preferred fracture plan, which is determined by the prevailing stress regime in the rock. Re-orientation of the fracture to the preferred fracture plane after initiating in the direction of the open tunnels will result in additional tortuosity. Such tortuosity is a primary cause of excessive injection pressure, premature screenout, and incomplete fracture stimulation treatment execution.
Thus, inadequately cleaned tunnels limit the outflow area through which injection fluids can flow; inhibit injection rates at a given injection pressure; impair fracture initiation and propagation; increase the flux rate per open perforation, causing unwanted, increased erosion; and increase the risk that solids bridging across the open perforations will eventually result in catastrophic loss of injectivity (also known as “screen out”). Further, it becomes very difficult to accurately predict the outflow area created by a given set of perforations and the discussed prior art methods do not remedy the uncertainties associated with damaged perforation tunnels.
Consequently, there is a need for a method of reducing the effects experienced when using conventional perforators in heterogeneous formations. There is also a need for a method of reducing the effects of plastic deformation in moderate to high strength rocks and enhancing perforation cleanup, preferably achieved as part of the primary perforating operation and not by introducing additional operation complexity or cost. Further, there is a need for a method of enhancing the parameters and effects of injection to enhance and stimulate the production of oil and gas.